Analog circuits utilize operational amplifiers or op-amps as a core components for their functioning. The ideal characteristics of the operational amplifier include infinite gain and infinite bandwidth with very high input impedance and zero output impedance. Factors such as gain, bandwidth and power consumption of the op-amp are major factors responsible for limiting the accuracy inside analog circuits. There are many techniques proposed for achieving a high gain with wide bandwidth characteristics. Gain enhancement or gain boosting is one such known technique.
FIG. 1 illustrates a schematic of a conventional operational amplifier 100 with single ended gain boosting amplifiers. This scheme is proposed in “A fast-settling CMOS op amp for SC circuits with 90-db dc gain” by K. Bult and G. Geelen, IEEE Journal of Solid State Circuits, vol. 25, pp. 1379-1394, December 1990, the disclosure of which is hereby incorporated by reference. This technique has enabled circuit designers to exploit the advantages of single-stage amplifiers with an adequate gain. Single ended gain boosting amplifiers have great flexibility in biasing and maintaining the saturation margins of cascode as well as current mirrors but at the cost of extra power as compared to fully differential gain boosting amplifiers because four single ended boosters are required as compared to two in case of fully differential ones. Since one input terminal of the single ended gain boosting amplifiers is fixed, so the gain available is similar in both cases and fully differential boosters are inherently faster because of absence of mirror poles. Being faster inherently, this saves power as compared to single ended boosting amplifiers. But it is difficult to bias cascode transistors with fully differential gain boosters in a way to ensure their saturation margins with process variations.
Another conventional technique is presented in U.S. Pat. No. 5,748,040, the disclosure of which is incorporated by reference. This circuit proposes a technique to adapt to the process variations but it requires very high common mode bandwidth of the fully differential boosting amplifiers so that common mode signals cannot interfere with differential signals. This results in a greater more power requirement.
Another conventional technique is presented in U.S. Pat. No. 5,442,318, the disclosure of which is incorporated by reference. This reference explains that the common mode gain of boosting amplifiers should also be minimized in order to reduce noise sensitivity.
The conventional techniques utilize greater power consumption with poor transient behaviors. Moreover, the conventional techniques limit the output swing, which is also a critical requirement in the case of the low voltage applications.
Therefore, there is a need of a methodology for the reduction of power consumption in fully differential gain boosted operational amplifiers. Such a methodology could provide enhancement of output swing characteristics, reduction of process dependency, lower power consumption and improvements in transient operation.